NAME
Crypt::SaltedHash - Perl interface to functions that assist in working
with salted hashes.
SYNOPSIS
use Crypt::SaltedHash;
my $csh = Crypt::SaltedHash->new(algorithm => 'SHA-1');
$csh->add('secret');
my $salted = $csh->generate;
my $valid = Crypt::SaltedHash->validate($salted, 'secret');
DESCRIPTION
The "Crypt::SaltedHash" module provides an object oriented interface to
create salted (or seeded) hashes of clear text data. The original
formalization of this concept comes from RFC-3112 and is extended by the
use of different digital agorithms.
ABSTRACT
Setting the data
The process starts with 2 elements of data:
* a clear text string (this could represent a password for instance).
* the salt, a random seed of data. This is the value used to augment a
hash in order to ensure that 2 hashes of identical data yield
different output.
For the purposes of this abstract we will analyze the steps within code
that perform the necessary actions to achieve the endresult hashes.
Cryptographers call this hash a digest. We will not however go into an
explanation of a one-way encryption scheme. Readers of this abstract are
encouraged to get information on that subject by their own.
Theoretically, an implementation of a one-way function as an algorithm
takes input, and provides output, that are both in binary form;
realistically though digests are typically encoded and stored in a
database or in a flat text or XML file. Take slappasswd5 for instance,
it performs the exact functionality described above. We will use it as a
black box compiled piece of code for our analysis.
In pseudocode we generate a salted hash as follows:
Get the source string and salt as separate binary objects
Concatenate the 2 binary values
Hash the concatenation into SaltedPasswordHash
Base64Encode(concat(SaltedPasswordHash, Salt))
We take a clear text string and hash this into a binary object
representing the hashed value of the clear text string plus the random
salt. Then we have the Salt value, which are typically 4 bytes of purely
random binary data represented as hexadecimal notation (Base16 as 8
bytes).
Using SHA-1 as the hashing algorithm, SaltedPasswordHash is of length 20
(bytes) in raw binary form (40 bytes if we look at it in hex). Salt is
then 4 bytes in raw binary form. The SHA-1 algorithm generates a 160 bit
hash string. Consider that 8 bits = 1 byte. So 160 bits = 20 bytes,
which is exactly what the algorithm gives us.
The Base64 encoding of the binary result looks like:
{SSHA}B0O0XSYdsk7g9K229ZEr73Lid7HBD9DX
Take note here that the final output is a 32-byte string of data. The
Base64 encoding process uses bit shifting, masking, and padding as per
RFC-3548.
A couple of examples of salted hashes using on the same exact clear-text
string:
slappasswd -s testing123
{SSHA}72uhy5xc1AWOLwmNcXALHBSzp8xt4giL
slappasswd -s testing123
{SSHA}zmIAVaKMmTngrUi4UlS0dzYwVAbfBTl7
slappasswd -s testing123
{SSHA}Be3F12VVvBf9Sy6MSqpOgAdEj6JCZ+0f
slappasswd -s testing123
{SSHA}ncHs4XYmQKJqL+VuyNQzQjwRXfvu6noa
4 runs of slappasswd against the same clear text string each yielded
unique endresult hashes. The random salt is generated silently and never
made visible.
Extracting the data
One of the keys to note is that the salt is dealt with twice in the
process. It is used once for the actual application of randomness to the
given clear text string, and then it is stored within the final output
as purely Base64 encoded data. In order to perform an authentication
query for instance, we must break apart the concatenation that was
created for storage of the data. We accomplish this by splitting up the
binary data we get after Base64 decoding the stored hash.
In pseudocode we would perform the extraction and verification
operations as such:
Strip the hash identifier from the Digest
Base64Decode(Digest, 20)
Split Digest into 2 byte arrays, one for bytes 0 – 20(pwhash), one for bytes 21 – 32 (salt)
Get the target string and salt as separate binary object
Concatenate the 2 binary values
SHA hash the concatenation into targetPasswordHash
Compare targetPasswordHash with pwhash
Return corresponding Boolean value
Our job is to split the original digest up into 2 distinct byte arrays,
one of the left 20 (0 - 20 including the null terminator) bytes and the
other for the rest of the data. The left 0 – 20 bytes will represent the
salted binary value we will use for a byte-by-byte data match against
the new clear text presented for verification. The string presented for
verification will have to be salted as well. The rest of the bytes (21 –
32) represent the random salt which when decoded will show the exact hex
characters that make up the once randomly generated seed.
We are now ready to verify some data. Let's start with the 4 hashes
presented earlier. We will run them through our code to extract the
random salt and then using that verify the clear text string hashed by
slappasswd. First, let's do a verification test with an erroneous
password; this should fail the matching test:
{SSHA}72uhy5xc1AWOLwmNcXALHBSzp8xt4giL Test123
Hash extracted (in hex): ef6ba1cb9c5cd4058e2f098d71700b1c14b3a7cc
Salt extracted (in hex): 6de2088b
Hash length is: 20 Salt length is: 4
Hash presented in hex: 256bc48def0ce04b0af90dfd2808c42588bf9542
Hashes DON'T match: Test123
The match failure test was successful as expected. Now let's use known
valid data through the same exact code:
{SSHA}72uhy5xc1AWOLwmNcXALHBSzp8xt4giL testing123
Hash extracted (in hex): ef6ba1cb9c5cd4058e2f098d71700b1c14b3a7cc
Salt extracted (in hex): 6de2088b
Hash length is: 20 Salt length is: 4
Hash presented in hex: ef6ba1cb9c5cd4058e2f098d71700b1c14b3a7cc
Hashes match: testing123
The process used for salted passwords should now be clear. We see that
salting hashed data does indeed add another layer of security to the
clear text one-way hashing process. But we also see that salted hashes
should also be protected just as if the data was in clear text form. Now
that we have seen salted hashes actually work you should also realize
that in code it is possible to extract salt values and use them for
various purposes. Obviously the usage can be on either side of the
colored hat line, but the data is there.
METHODS
new( [%options] )
Returns a new Crypt::SaltedHash object. Possible keys for *%options*
are:
* *algorithm*: It's also possible to use common string
representations of the algorithm (e.g. "sha256", "SHA-384"). If
the argument is missing, SHA-1 will be used by default.
* *salt*: You can specify your on salt. You can either specify it
as a sequence of charactres or as a hex encoded string of the
form "HEX{...}". If the argument is missing, a random seed is
provided for you (recommended).
* *salt_len*: By default, the module assumes a salt length of 4
bytes (or 8, if it is encoded in hex). If you choose a different
length, you have to tell the *validate* function how long your
seed was.
add( $data, ... )
Logically joins the arguments into a single string, and uses it to
update the current digest state. For more details see Digest.
salt_bin()
Returns the salt in binary form.
salt_hex()
Returns the salt in hexadecimal form ('HEX{...}')
generate()
Generates the seeded hash. Uses the *clone*-method of Digest before
actually performing the digest calculation, so adding more cleardata
after a call of *generate* to an instance of *Crypt::SaltedHash* has
the same effect as adding the data before the call of *generate*.
validate( $hasheddata, $cleardata, [$salt_len] )
Validates a hasheddata previously generated against cleardata.
*$salt_len* defaults to 4 if not set. Returns 1 if the validation is
successful, 0 otherwise.
obj()
Returns a handle to Digest object.
FUNCTIONS
*none yet.*
SEE ALSO
Digest, MIME::Base64
AUTHOR
Sascha Kiefer, esskar@cpan.org
ACKNOWLEDGMENTS
The author is particularly grateful to Andres Andreu for his article:
Salted hashes demystified - A Primer
()
COPYRIGHT AND LICENSE
Copyright (C) 2005 Sascha Kiefer
This library is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.